Safety glasses are widely used for windows of automobiles, airplanes, and buildings. The safety glasses are referred to as laminated glasses. The safety glass has an interlayer containing synthetic resins inserted between two sheets of glass plates. In order to avoid obstruction of visibility, a transparent synthetic resin film is generally used as the above-described interlayer.
Windshields of automobiles preferably block transmission of sunlight within vision of drivers. Therefore, a colored belt for blocking sunlight is sometimes formed in the upper portion of the windshield. Such colored belts can be formed by attachment of a colored film on the interior surface or the outside surface of the windshield, or formation of a colored belt in the above-described interlayer.
Formation of the colored belt in the interlayer eliminates the necessity for additional attachment of the colored film to windshields, thereby avoiding problems, such as separation of the colored film.
Various interlayers have been proposed as interlayers having the colored belt provided therein. For example, known are interlayers with a three layered structure sandwiching a transparent synthetic resin film having a colored belt that is inserted into a pair of transparent synthetic resin films and that is printed in the formation area of the colored belt. Incidentally, interlayers with a colored belt having a partial three layered structure obtained by formation of a colored belt layer between a pair of outermost layers including a transparent synthetic resin by extrusion molding are also known.
In use of the above-described transparent synthetic resin film having a printed colored belt, it was necessary that the transparent synthetic resin film having the colored belt was sandwiched between two sheets of transparent resin films, and then these films was fused with each other by roll pressing etc. The interlayers obtained by roll pressing, however, allowed easy involvement of air bubbles remaining within the interface between the laminated films.
Incidentally, the interface between the thermally adhered films failed to develop sufficient adhesive strength. Furthermore, the interface obtained by thermal adhesion between the transparent synthetic resin film and the colored belt allowed occurrence of opaque lines in the delivery direction of the interlayer for production.
Following patent document 1 discloses an example of a method of forming an interlayer with a colored belt having partial three layered structure using extrusion molding method. In the document, a probe is inserted in a portion in which the direction of flow of the transparent main raw material resin composition is converted in a shape of a character L in production of an interlayer by extrusion molding using an L die. Then, injection of a coloring resin into the main raw material resin from an orifice of probe forms a partially colored belt in the interlayer containing the transparent main raw material resin.
Injection of coloring resins from an orifice into the main raw material resin gives a consequent blurry boundary line in one edge line of the colored belt. Accordingly, this method can form a gradation in which the grade of coloring gradually decreases in one edge line of the colored belt. Therefore, this method can form the above-described gradation in the edge line of the colored belt in safety glass, and can keep the vision satisfactorily even in the case where the colored belt comes into the vision of the driver.    Patent document 1: JP,7-40413,A